Field of the Invention
The present invention relates to a method for producing a semiconductor device and to a semiconductor device.
Description of the Related Art
For semiconductor integrated circuits, in particular, integrated circuits employing MOS transistors, the degree of integration has been continuously increased. With this increase in the degree of integration, the size of MOS transistors in such integrated circuits has been reduced to the order of nanometers. In such small MOS transistors, leak current is difficult to suppress. Thus, from the standpoint of ensuring a sufficiently large current, reduction in the circuit area is difficult to achieve, which has been problematic. In order to address this problem, a Surrounding Gate Transistor (hereafter, referred to as an “SGT”) has been proposed, the SGT having a structure in which a source, a gate, and a drain are disposed so as to be perpendicular to a substrate and a gate electrode is disposed so as to surround a pillar-shaped semiconductor layer (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2-71556, 2-188966, and 3-145761).
In existing SGT production methods, a mask for defining a silicon pillar is used to form the silicon pillar including a pillar-shaped nitride-film hard mask; a mask for defining a planar silicon layer is used to form the planar silicon layer under the silicon pillar; and a mask for defining a gate line is used to form the gate line (for example, refer to Japanese Unexamined Patent Application Publication No. 2009-182317).
In other words, three masks are used to form the silicon pillar, the planar silicon layer, and the gate line.
In the production of prior art MOS transistors, in order to successfully perform a metal-gate process and a high-temperature process, a metal-gate last process of performing the high-temperature process and subsequently forming a metal gate is employed (IEDM2007 K. Mistry et. al, pp 247-250). Specifically, a gate is formed of polysilicon; an interlayer insulating film is then deposited; chemical mechanical polishing is then performed to expose the polysilicon gate; the polysilicon gate is etched; and metal is then deposited. Thus, also for SGTs, in order to successfully perforin the metal-gate process and the high-temperature process, the metal-gate last process of performing the high-temperature process and subsequently forming the metal gate needs to be employed.
When metal is deposited to fill a hole in which the upper portion is narrower than the lower portion, the upper portion of the hole is first filled with the metal, so that the lower portion is left unfilled.
Prior art MOS transistors employ a first insulating film in order to decrease the parasitic capacitance between the gate line and the substrate. For example, in FINFET (IEDM2010 CC. Wu, et. al, 27.1.1-27.1.4), a first insulating film is formed around a fin-shaped semiconductor layer and the first insulating film is subjected to etch back to expose the fin-shaped semiconductor layer, so that the parasitic capacitance between the gate line and the substrate is decreased. Thus, SGTs also need to employ a first insulating film in order to decrease the parasitic capacitance between the gate line and the substrate. Since SGTs include a fin-shaped semiconductor layer and also a pillar-shaped semiconductor layer, how to form the pillar-shaped semiconductor layer needs to be considered.
When the silicon pillar is narrow, since silicon has a density of 5×1022 atoms/cm3, it becomes difficult to make impurities be present within the silicon pillar.
For existing SGTs, it has been proposed that the channel concentration is set to a low impurity concentration of 1017 cm−3 or less and the work function of the gate material is changed to adjust the threshold voltage (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-356314).
In a planar MOS transistor, sidewalls on LDD regions are formed of polysilicon of the same conductivity type as that of low-concentration layers, so that surface carriers of the LDD regions are induced by the work-function difference, enabling a decrease in the impedance of the LDD regions, compared with oxide-film-sidewall LDD-type MOS transistors (for example, refer to Japanese Unexamined Patent Application Publication No. 11-297984). This publication discloses that the polysilicon sidewalls are electrically insulated from the gate electrode. The publication also discloses that, in figures, insulation of the polysilicon sidewalls from the source and the drain is achieved with an interlayer insulating film.